1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a high density recording medium having excellent physical and electromagnetic conversion characteristics.
2. Description of the Prior Art
As ferromagnetic powders which have been employed for magnetic recording media, there are, for instance, .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4 and CrO.sub.2. However, the magnetic characteristics of these ferromagnetic powders, such as the coercive force and the maximum residual magnetic flux density, are inadequate for high density recording with high sensitivity, and they are not suitable for magnetic recording of a signal having a short recording wavelength of at most about 1 .mu.m or for magnetic recording with a narrow track width.
As the requirements for such magnetic recording media have become to be severe, ferromagnetic powders having characteristics suitable for high density recording have been developed or proposed. As such magnetic powders, there may be mentioned metals or alloys such as Fe, Co, Fe-Co, Fe-Co-Ni and Co-Ni, or alloys of these materials with Al, Cr or Si. A magnetic recording layer employing such an alloy powder, is required to have a high coercive force and a high residual magnetic flux density for the purpose of the high density recording, and it is required to select a process for the preparation or an alloy composition so that the above-mentioned magnetic powder meets these requirements.
The present inventors have prepared magnetic recording media by employing various alloy powders, and have found that a magnetic recording medium with a noise level being sufficiently low and suitable for high density short wave recording is obtainable when the specific surface area as measured by BET method is at least 48 m.sup.2 /g, the coercive force of the magnetic layer is at least 1000 Oe, and the surface roughness of the magnetic layer is at most 0.08 .mu.m as a R.sub.20 value (an average value of 20 values) with a cut off of 0.17 mm by Talystep method. However, from a further study, it has been found that when the magnetic recording medium is wound or permitted to run e.g. as a video tape, the desired effects are not necessarily obtainable even when the magnetic layer satisfy the above-mentioned requirements, and that the properties of the rear side of the plastic film (usually polyester film) supporting the magnetic layer, have a close relationship with the physical and electric characteristics of the magnetic recording medium.
Under the circumstances, the present inventors have tried to form a backing layer to the rear side of the plastic base film. As a result, it has been found that the S/N ratio of the magnetic recording medium is affected not only by the specific surface area of the alloy powder used for the magnetic layer as measured by BET method and the surface roughness of the magnetic layer, but also by the surface roughness of the backing layer. Further, because of additional problems such as the cinching phenomenon (the loosening of the tape winding when the tape has been stopped abruptly), the abrasion or friction of the backing layer and the adhesion of the magnetic layer with the backing layer, it is impossible to obtain a magnetic recording medium having superior electromagnetic conversion characteristics, running characteristics and durability, without using a proper backing layer.
It has been found that depending upon the nature of the backing layer, tightening of the tape winding is likely to take place when the magnetic tape is wound on the reel, and the surface roughness of the backing layer is likely to be transferred to the magnetic layer, whereby the magnetic layer surface will be roughened, and the desired characteristic of the backing layer will not adequately be utilized. The present inventors have employed a thermosetting composition comprising a vinyl chloride-vinyl acetate-vinyl alcohol copolymer, a polyurethane resin and a polyisocyanate, or a thermosetting composition comprising such materials and nitrocellulose, as binder for the backing layer, whereby excellent effects have been obtained such as a decrease of the abrasion of the backing surface and the magnetic surface, a decrease of the dropouts and a decrease of the cinching phenomenon and friction. However, there is a recent trend that a plastic film such as a polyethylene terephthalate, polyethylene naphthalate, polyimide or polyamide film as a base film for the magnetic tape, tends to be as thin as about 11 .mu.m or less, and accordingly when the tape is wound, there is an increasing tendency for tightening of the tape winding, whereby the roughness of the backing layer tends to be transferred onto the magnetic layer surface, thus leading to a decrease of the output.